Building block



FIG. 1 is an isometric view of a building block according to the invention;

FIG. 2 is a side elevation of a building block according to the invnetion;

FIG. 3 is a top plan view of a building block according to the invention; and

FIG. 4 is an end elevation of a building block according to the invention. 

The ornamental design for a building block, as shown. 